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An Edusat Lecture on

by:MOHINDER

01-02-2013

KUMAR

Sr. Lecturer Civil Engg.
Govt. Polytechnic College,
BATALA
Contour An imaginary line on the ground
surface joining the points of equal elevation is
known as contour.
In other words, contour is a line in which the
ground surface is intersected by a level
surface obtained by joining points of equal
elevation. This line on the map represents a
contour and is called contour line.

CONTOURING

22
Contour Map
A map showing contour lines is known as
Contour map.
A contour map gives an idea of the altitudes of
the surface features as well as their relative
positions in plan serves the purpose of both, a
plan and a section.

CONTOURING

3
3
Contouring
The process of tracing contour lines on
the surface of the earth is called
Contouring.

CONTOURING

4
3
PURPOSE OF CONTOURING
Contour survey is carried out at the starting of
any engineering project such as a road, a
railway, a canal, a dam, a building etc.
i) For preparing contour maps in order to select
the most economical or suitable site.
ii) To locate the alignment of a canal so that it
should follow a ridge line.
iii) To mark the alignment of roads and railways
so that the quantity of earthwork both in
cutting and filling should be minimum.
CONTOURING

5
4
PURPOSE OF CONTOURING (contd.)
iv) For getting information about the ground
whether it is flat, undulating or
mountainous.
v) To find the capacity of a reservoir and
volume of earthwork especially in a
mountainous region.
vi) To trace out the given grade of a particular
route.
vii)To locate the physical features of the ground
such as a pond depression, hill, steep or
small slopes.
CONTOURING

6
CONTOUR INTERVAL
The constant vertical distance between two
consecutive contours is called the contour
interval.
HORIZONTAL EQUIVALENT
The horizontal distance between any two
adjacent contours is called as horizontal
equivalent.
The contour interval is constant between
the consecutive contours while the horizontal
equivalent is variable and depends upon the
slope of the ground.
CONTOURING

7
FACTORS ON WHICH CONTOUR INTERVAL DEPENDS
The contour interval depends upon the following
factors:i) The Nature of the Ground In flat and
uniformly sloping country, the contour
interval is small , but in broken and
mountainous region the contour interval
should be large otherwise the contours will
come too close to each other.

CONTOURING

8
FACTORS ON WHICH CONTOUR INTERVAL DEPENDS
ii) The Purpose and extent of the survey.
Contour interval is small if the area to be
surveyed is small and the maps are required
to be used for the design work or for
determining the quantities of earth work etc.
while wider interval shall have to be kept for
large areas and comparatively less important
works.
CONTOURING

9
FACTORS ON WHICH CONTOUR INTERVAL DEPENDS
iii) The Scale of the Map. The contour interval
should be in the inverse ratio to the scale of
the map i.e. the smaller the scale, the greater
is the contour interval.
iv) Time and Expense of Field and Office work.
The smaller the interval, the greater is the
amount of field-work and plotting work.
CONTOURING

10
COMMON VALUES OF THE CONTOUR INTERVAL
The following are the common values of the
contour interval adopted for various purposes:i) For large scale maps of flat country, for
building sites, for detailed design work and
for calculation of quantities of earth work;
0.2 to 0.5 m.

CONTOURING

11
COMMON VALUES OF THE CONTOUR INTERVAL
ii) For reservoirs and town planning schemes;
0.5 to 2m.
iii) For location surveys.

2 to 3m.

iv) For small scale maps of broken country and
general topographic work; 3m,5m,10m,or
25m.

CONTOURING

12
CHARACTERISTICS OF CONTOURS
i) All points in a contour line have the same
elevation.
ii) Flat ground is indicated where the
…contours are widely separated and steepslope where they run close together.
iii) A uniform slope is indicated when the
contour lines are uniformly spaced and
iv) A plane surface when they are straight,
parallel and equally spaced.

CONTOURING

13
CHARACTERISTICS OF CONTOURS
v) A series of
closed contour
lines on the
map represent
a hill , if the
higher values
are inside

80

75
70
65
HILL

60

60
65
70
75
80

A HILL
CHARACTERISTICS OF CONTOURS
80

vi) A series of
closed contour
lines on the
map indicate a
depression if
the
higher
values
are
outside

75
70

DEPRESSION

65
60

70
60
65
70
75
80

A DEPRESSION

CONTOURING

15
CHARACTERISTICS OF CONTOURS
vii) Contour line cross ridge or valley line at
right angles.
100
If the higher
90
values are inside
80
the bend or loop
70
in the contour, it
60
indicates
a
50
Ridge.

RIDGE LINE

CONTOURING

16
CHARACTERISTICS OF CONTOURS
vii) Contour line cross ridge or valley line at
right angles.
If the higher
values are
100
outside the
90
bend,
it
80
represents
70
60
a Valley
50

VALLEY LINE

CONTOURING

17
CHARACTERISTICS OF CONTOURS
viii).
Contours cannot end anywhere but close on
themselves either within or outside the limits of
the map.

CONTOURING

18
CHARACTERISTICS OF CONTOURS
ix).
Contour lines cannot
merge or cross one
another
on map
except in the case of
an overhanging cliff.

40
30
20
10

40

30 20 10

OVERHANGING CLIFF

CONTOURING

19
CHARACTERISTICS OF CONTOURS
x) Contour lines
50
never run into one
40
another except in
30
the case of a
20
vertical cliff. In
10
this case ,several
contours coincide
and the horizontal
equivalent becomes
zero.

VERTICA
CLIFF

50
10 20 30 40 50

OVERHANGING CLIFF

CONTOURING

20
CHARACTERISTICS OF CONTOURS
X
Depressions
between summits is
called a saddle. It is
represented by four
SADDLE
70
sets of contours as 70
80
80
90
shown. It represents 90
90
a dip in a ridge or
100
110
the junction of two
ridges. And in the
case of a mountain Line passing through the
range ,it takes the saddles and summits gives
water shed line.
form of a pass .
CONTOURING

21
METHODS OF CONTOURING
There are mainly two methods of locating contours:(1)Direct Method and (2) Indirect Method.

Direct Method:
In this method, the contours to be
located are directly traced out in
the field by locating and marking
a number of points on each
contour. These points are then
surveyed and plotted on plan and
the contours drawn through
them.
CONTOURING

B.M

50
48

46

DIRECT METHOD OF
CONTOURING

22
METHODS OF CONTOURING
Direct Method:

•This method is most accurate but very slow
and tedious as a lot of time is wasted in
searching points of the same elevation for a
contour.
•This is suitable for small area and where great
accuracy is required

CONTOURING

23
Procedure: To start with, a temporary B.M is
established near the area to be surveyed with
reference to a permanent B.M by fly leveling. The
level is then set up in such a position so that the
maximum number of points can be commanded
from the instrument station. The height of
instrument is determined by taking a back sight
on the B.M. and adding it to the R.L. of bench
mark. The staff reading required to fix points on
the various contours is determined by
subtracting the R.L. of each of the contours from
the height of instrument.
CONTOURING

24
Example:
If the height of instrument is 82.48m.,
then the staff readings required to locate 82,
81 and 80m contours are 0.48, 1.48 and 2.48m
respectively. The staff is held on an
approximate position of point and then
moved up and down the slope until the
desired reading is obtained. The point is
marked with a peg.
CONTOURING

25
Example(contd.):

Similarly various other points are marked
on each contour. The line joining all these
points give the required contour. It may be
noted that one contour is located at a time.
Having fixed the contours within the range
of the instrument, the level is shifted and set
up in a new position.

CONTOURING

26
Procedure (Contd…..)
The new height of instrument and the
required staff readings are then calculated in a
similar manner and the process repeated till all
the contours are located. The positions of the
contour points are located suitably either
simultaneous with levelling or afterwards. A
theodolite , a compass or a plane table
traversing is usually adopted for locating these
points. The points are then plotted on the plan
and the contours drawn by joining the
corresponding points by dotted curved lines.
CONTOURING

27
Direct Method By Radial Lines Lines:
This method is suitable
for small areas, where a
single point in the centre
can command the whole
area. Radial lines are
laid out from the
common
centre
by
theodolite or compass
and their positions are
fixed up by horizontal
angles and bearings.
CONTOURING

70
65
60
55

Fig. RADIAL LINES
METHOD OF CONTOURING

28
Direct Method By Radial Lines Lines (contd.):
Temporary bench marks are first
established at the centre and near the ends of
the radial lines .The contour points are then
located and marked on these lines and their
positions are determined by measuring their
distances along the radial lines. They are then
plotted on the plan and the contours drawn by
joining all the corresponding points with the
help of a plane table instrument.
CONTOURING

29
2. Indirect Method:

In this method the points located and
surveyed are not necessarily on the contour lines
but the spot levels are taken along the series of
lines laid out over the area .The spot levels of the
several representative points representing hills,
depressions, ridge and valley lines and the
changes in the slope all over the area to be
contoured are also observed. Their positions are
then plotted on the plan and the contours drawn
by interpolation. This method of contouring is
also known as contouring by spot levels.
CONTOURING

30
SQUARE METHOD

CONTOURING

31
This method is commonly employed in all
kinds of surveys as this is cheaper, quicker
and less tedious as compared to direct
method. There are mainly three method of
contouring in indirect method:
(i) By Squares. In this method, the whole
area is divided into number of squares, the
side of which may vary from 5m to 30m
depending upon the nature of the ground
and the contour interval. The square need
not be of the same size throughout.
CONTOURING

31
2. Indirect Method: (Square Method)
The corners of the squares are pegged out
and the reduced levels of these points are
determined with a level.
100.4

13

14

15

97.60

96.05

94.40

16
98.00

98.00
98,00

9

10

11

97.45

6

98.75

99.40

12

98.65

5

99.25

7

99.60

8

97.80

89.55

98.0

99.0

99.0
100.90

1
2
3
4
SQURES LAID ON GROUND

99.70

99.85

98.45

CONTOURS INTERPOLATED

CONTOURING

32
2. Indirect Method: (Square Method)
The
important
points within the
squares may be
taken when required
and
located
by
measurements from
the corners. The
squares are plotted
and the reduced
levels of the corners
are written on the
plan.

100.4

97.60

96.05

94.40

98.00

98.00
98,00

97.45

99.25

98.65

99.60

98.75

97.80

99.40

89.55

98.0

99.0

99.0
100.90

99.70

99.85

98.45

SQUARE METHOD

CONTOURING

33
2. Indirect Method:

By Cross- Sections: This method is most suitable for
the survey of long narrow strips such as a road,
railway or canal etc.
70

RD 580 70.6

RD 560 70.8

70

69.1

69

69

68.8

69.1

70.8

70.2

69.1

70.4

70.5

70.8

66.3

70.6

70.8

71
RD 540

71.2

67
68

71

69
RD 520 71.6

71.2

70.6

70

71

CONTOURING

71

72.4

71.7

Fig. X-Section Method
34
2. Indirect Method:
By Cross- Sections: This method is most suitable
for the surveys of long narrow strips such as a
road, railway or canal etc. Cross sections are
run transverse to the centre line of the work and
representative points are marked along the lines
of cross-section. The cross-section lines need not
necessarily be at right angles to the centre line of
the work. This may be inclined at any angle to
the centre line if necessary. The spacing of the
cross-sections depends upon the topography of
the country and the nature of the survey.
CONTOURING

35
2. Indirect Method:
By Cross- Sections:
The common value is 20 to 30 m in
hilly country and 100m in flat country. The
levels of the points along the section lines are
plotted on the plan and the contours are then
interpolated as usual as shown in the fig.

CONTOURING

36
2. Indirect Method:

Fig. Stadia Wires at Diaphragm

(iii) By Tacheometric method:
A techeometer is a transit theodolite having a
diaphragm fitted with two stadia wires, one above
and other below the central wire. The horizontal
distance between the instrument and staff station
may be determined by multiplying the difference of
the staff readings of the upper and lower stadia wires
with the stadia constant of the instrument, which is
usually 100.Thus the techeometer is used for both the
vertical as well as horizontal measurements.
CONTOURING

37
2. Indirect Method:
iii) By Tacheometric method (contd…):
This method is most suitable in hilly areas as the
number of stations which can be commanded by a
techeometer is far more than those by a level and thus
the number of instrument settings are considerably
reduced. A number of radial lines are laid out at a
known angular interval and representative points are
marked by pegs along these radial lines. Their
elevations and distances are then calculated and plotted
on the plan and the contour lines are then interpolated.

CONTOURING

38
INTERPOLATION OF CONTOURS
The process of spacing the
contours proportionally between the plotted
ground – points is termed as interpolation
of contours .
This becomes necessary in the case
of indirect contouring as only the spot
levels are taken in this method.
While interpolation of contours the ground
between any two points is assumed to be
uniformly sloping.
CONTOURING

39
INTERPOLATION OF CONTOURS
There are three main methods of interpolation:
i) By Estimation: The position of the contour
points between ground - points are
estimated roughly and the contours are then
drawn through these points. This is a rough
method and is suitable for small scale maps.
ii) By arithmetical calculation: This is very
tedious but accurate method and is used for
small areas where accurate results are
necessary. The contours are interpolated as
under:
CONTOURING

40
INTERPOLATION OF CONTOURS
ii) By arithmetical calculation:
Suppose A and B are two points at a distance
of 30 m and the reduced level of A and B are 25.45m and
27.54m respectively .Taking the contour interval as 1m,
26 and 27 m contours may be interpolated in between A
and B. The difference of level between A and B is
2.09m.the difference of level between A and 26m,and A
and 27m is 0.55mand 1.55 m respectively.
Therefore the horizontal distance between A and 26 m
contour =0.55/2.09 x 30m and
Between A and 27 m contour =1.55/2.09 x 30m.
These distances are then plotted to scale on the map.
CONTOURING

41
INTERPOLATION OF CONTOURS
Graphical method of (iii) By Graphical method:
interpolation
is 65
simpler as compared
B
62.5m
to
arithmetical
2
method and also the 60
60m
results obtained are
accurate. Out of
several
graphical 55
1
55m
methods, the most
common is as given
A 51.5m
below:
50
CONTOURING

42
Graphical method:
As shown in the fig. suppose the contour
interval is 5m, then on a piece of tracing cloth, a
number of parallel lines spaced at 0.5 m (usually
1/10th of the contour interval) are drawn. Every
tenth line being made thick.
Suppose it is required to interpolate contours
between two points A and B of elevation 51.5m
and 62.5m respectively.
CONTOURING

43
INTERPOLATION OF CONTOURS
(iii) By Graphical method:
If the bottom line represents an
elevation of 50m. Then the successive thick lines
will represent 55m, 60m and 65m, etc. Place the
tracing cloth so that the point A is on the third line
from the bottom, now move the tracing cloth until
B is on the fifth line above the 60m thick line. The
intersection of the thick lines 1 and 2 representing
elevations of 55m and 60 m and the line AB give
the position of the points on the 55m and 60m
contours respectively and are pricked through on
the plan with a pin.
CONTOURING

44
DRAWING THE CONTOUR LINES
Contour lines are drawn as fine and smooth free
hand curved lines. Sometimes they are represented by
broken lines .They are inked in either in black or brown
colour. A drawing pen gives a better line than a writing
pen and French curves should be used as much as
possible .Every fifth contour is made thicker than the
rest.
The elevation of contours must be written in a
uniform manner, either on the higher side or in a gap left
in the line .When the contour lines are very long, their
elevations are written at two or three places along the
contour .In the case of small scale maps, it is sufficient to
figure every fifth contour.
CONTOURING

45
USES OF CONTOUR MAP
(i) A contour map furnishes information regarding the features
of the ground , whether it is flat, undulating or mountainous.
(ii) From a contour map , sections may be easily drawn in any
direction
(iii) Intervisibility between two ground points plotted on map
can be ascertained
(iv) It enables an engineer to approximately select the most
economical or suitable site for an engineering project such as
a road, a railway, a canal or a pipe line etc.
(v) A route of a given grade can be traced on the map.
(vi) Catchment area and capacity of a reservoir may be
determined from the contour map.
(vii) Contour map may be used to determine the quantities of
earth work.
CONTOURING

45
HOME ASSIGNMENT
Q.No.1. (a) Define contour.
(b) What is a contour interval and on what factors does it depend?.
Q.No.2.(a) What is difference between a contour interval and horizontal
…………….equivalent?.

(b) Suggest contour intervals for following :(i) A hill Survey (ii) A city Survey (iii) survey of a dam site.
Q.No.3. Describe with neat sketches the characteristics of contours.

Q.No.4. Show contours to represent the following :(i) A Hill (ii) A Depression (iii) A valley (iv) A vertical cliff (v) A saddle.
Q.No.5. Describe various methods of interpolation of contours.

CONTOURING

46

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Contouring

  • 1. An Edusat Lecture on by:MOHINDER 01-02-2013 KUMAR Sr. Lecturer Civil Engg. Govt. Polytechnic College, BATALA
  • 2. Contour An imaginary line on the ground surface joining the points of equal elevation is known as contour. In other words, contour is a line in which the ground surface is intersected by a level surface obtained by joining points of equal elevation. This line on the map represents a contour and is called contour line. CONTOURING 22
  • 3. Contour Map A map showing contour lines is known as Contour map. A contour map gives an idea of the altitudes of the surface features as well as their relative positions in plan serves the purpose of both, a plan and a section. CONTOURING 3 3
  • 4. Contouring The process of tracing contour lines on the surface of the earth is called Contouring. CONTOURING 4 3
  • 5. PURPOSE OF CONTOURING Contour survey is carried out at the starting of any engineering project such as a road, a railway, a canal, a dam, a building etc. i) For preparing contour maps in order to select the most economical or suitable site. ii) To locate the alignment of a canal so that it should follow a ridge line. iii) To mark the alignment of roads and railways so that the quantity of earthwork both in cutting and filling should be minimum. CONTOURING 5 4
  • 6. PURPOSE OF CONTOURING (contd.) iv) For getting information about the ground whether it is flat, undulating or mountainous. v) To find the capacity of a reservoir and volume of earthwork especially in a mountainous region. vi) To trace out the given grade of a particular route. vii)To locate the physical features of the ground such as a pond depression, hill, steep or small slopes. CONTOURING 6
  • 7. CONTOUR INTERVAL The constant vertical distance between two consecutive contours is called the contour interval. HORIZONTAL EQUIVALENT The horizontal distance between any two adjacent contours is called as horizontal equivalent. The contour interval is constant between the consecutive contours while the horizontal equivalent is variable and depends upon the slope of the ground. CONTOURING 7
  • 8. FACTORS ON WHICH CONTOUR INTERVAL DEPENDS The contour interval depends upon the following factors:i) The Nature of the Ground In flat and uniformly sloping country, the contour interval is small , but in broken and mountainous region the contour interval should be large otherwise the contours will come too close to each other. CONTOURING 8
  • 9. FACTORS ON WHICH CONTOUR INTERVAL DEPENDS ii) The Purpose and extent of the survey. Contour interval is small if the area to be surveyed is small and the maps are required to be used for the design work or for determining the quantities of earth work etc. while wider interval shall have to be kept for large areas and comparatively less important works. CONTOURING 9
  • 10. FACTORS ON WHICH CONTOUR INTERVAL DEPENDS iii) The Scale of the Map. The contour interval should be in the inverse ratio to the scale of the map i.e. the smaller the scale, the greater is the contour interval. iv) Time and Expense of Field and Office work. The smaller the interval, the greater is the amount of field-work and plotting work. CONTOURING 10
  • 11. COMMON VALUES OF THE CONTOUR INTERVAL The following are the common values of the contour interval adopted for various purposes:i) For large scale maps of flat country, for building sites, for detailed design work and for calculation of quantities of earth work; 0.2 to 0.5 m. CONTOURING 11
  • 12. COMMON VALUES OF THE CONTOUR INTERVAL ii) For reservoirs and town planning schemes; 0.5 to 2m. iii) For location surveys. 2 to 3m. iv) For small scale maps of broken country and general topographic work; 3m,5m,10m,or 25m. CONTOURING 12
  • 13. CHARACTERISTICS OF CONTOURS i) All points in a contour line have the same elevation. ii) Flat ground is indicated where the …contours are widely separated and steepslope where they run close together. iii) A uniform slope is indicated when the contour lines are uniformly spaced and iv) A plane surface when they are straight, parallel and equally spaced. CONTOURING 13
  • 14. CHARACTERISTICS OF CONTOURS v) A series of closed contour lines on the map represent a hill , if the higher values are inside 80 75 70 65 HILL 60 60 65 70 75 80 A HILL
  • 15. CHARACTERISTICS OF CONTOURS 80 vi) A series of closed contour lines on the map indicate a depression if the higher values are outside 75 70 DEPRESSION 65 60 70 60 65 70 75 80 A DEPRESSION CONTOURING 15
  • 16. CHARACTERISTICS OF CONTOURS vii) Contour line cross ridge or valley line at right angles. 100 If the higher 90 values are inside 80 the bend or loop 70 in the contour, it 60 indicates a 50 Ridge. RIDGE LINE CONTOURING 16
  • 17. CHARACTERISTICS OF CONTOURS vii) Contour line cross ridge or valley line at right angles. If the higher values are 100 outside the 90 bend, it 80 represents 70 60 a Valley 50 VALLEY LINE CONTOURING 17
  • 18. CHARACTERISTICS OF CONTOURS viii). Contours cannot end anywhere but close on themselves either within or outside the limits of the map. CONTOURING 18
  • 19. CHARACTERISTICS OF CONTOURS ix). Contour lines cannot merge or cross one another on map except in the case of an overhanging cliff. 40 30 20 10 40 30 20 10 OVERHANGING CLIFF CONTOURING 19
  • 20. CHARACTERISTICS OF CONTOURS x) Contour lines 50 never run into one 40 another except in 30 the case of a 20 vertical cliff. In 10 this case ,several contours coincide and the horizontal equivalent becomes zero. VERTICA CLIFF 50 10 20 30 40 50 OVERHANGING CLIFF CONTOURING 20
  • 21. CHARACTERISTICS OF CONTOURS X Depressions between summits is called a saddle. It is represented by four SADDLE 70 sets of contours as 70 80 80 90 shown. It represents 90 90 a dip in a ridge or 100 110 the junction of two ridges. And in the case of a mountain Line passing through the range ,it takes the saddles and summits gives water shed line. form of a pass . CONTOURING 21
  • 22. METHODS OF CONTOURING There are mainly two methods of locating contours:(1)Direct Method and (2) Indirect Method. Direct Method: In this method, the contours to be located are directly traced out in the field by locating and marking a number of points on each contour. These points are then surveyed and plotted on plan and the contours drawn through them. CONTOURING B.M 50 48 46 DIRECT METHOD OF CONTOURING 22
  • 23. METHODS OF CONTOURING Direct Method: •This method is most accurate but very slow and tedious as a lot of time is wasted in searching points of the same elevation for a contour. •This is suitable for small area and where great accuracy is required CONTOURING 23
  • 24. Procedure: To start with, a temporary B.M is established near the area to be surveyed with reference to a permanent B.M by fly leveling. The level is then set up in such a position so that the maximum number of points can be commanded from the instrument station. The height of instrument is determined by taking a back sight on the B.M. and adding it to the R.L. of bench mark. The staff reading required to fix points on the various contours is determined by subtracting the R.L. of each of the contours from the height of instrument. CONTOURING 24
  • 25. Example: If the height of instrument is 82.48m., then the staff readings required to locate 82, 81 and 80m contours are 0.48, 1.48 and 2.48m respectively. The staff is held on an approximate position of point and then moved up and down the slope until the desired reading is obtained. The point is marked with a peg. CONTOURING 25
  • 26. Example(contd.): Similarly various other points are marked on each contour. The line joining all these points give the required contour. It may be noted that one contour is located at a time. Having fixed the contours within the range of the instrument, the level is shifted and set up in a new position. CONTOURING 26
  • 27. Procedure (Contd…..) The new height of instrument and the required staff readings are then calculated in a similar manner and the process repeated till all the contours are located. The positions of the contour points are located suitably either simultaneous with levelling or afterwards. A theodolite , a compass or a plane table traversing is usually adopted for locating these points. The points are then plotted on the plan and the contours drawn by joining the corresponding points by dotted curved lines. CONTOURING 27
  • 28. Direct Method By Radial Lines Lines: This method is suitable for small areas, where a single point in the centre can command the whole area. Radial lines are laid out from the common centre by theodolite or compass and their positions are fixed up by horizontal angles and bearings. CONTOURING 70 65 60 55 Fig. RADIAL LINES METHOD OF CONTOURING 28
  • 29. Direct Method By Radial Lines Lines (contd.): Temporary bench marks are first established at the centre and near the ends of the radial lines .The contour points are then located and marked on these lines and their positions are determined by measuring their distances along the radial lines. They are then plotted on the plan and the contours drawn by joining all the corresponding points with the help of a plane table instrument. CONTOURING 29
  • 30. 2. Indirect Method: In this method the points located and surveyed are not necessarily on the contour lines but the spot levels are taken along the series of lines laid out over the area .The spot levels of the several representative points representing hills, depressions, ridge and valley lines and the changes in the slope all over the area to be contoured are also observed. Their positions are then plotted on the plan and the contours drawn by interpolation. This method of contouring is also known as contouring by spot levels. CONTOURING 30
  • 32. This method is commonly employed in all kinds of surveys as this is cheaper, quicker and less tedious as compared to direct method. There are mainly three method of contouring in indirect method: (i) By Squares. In this method, the whole area is divided into number of squares, the side of which may vary from 5m to 30m depending upon the nature of the ground and the contour interval. The square need not be of the same size throughout. CONTOURING 31
  • 33. 2. Indirect Method: (Square Method) The corners of the squares are pegged out and the reduced levels of these points are determined with a level. 100.4 13 14 15 97.60 96.05 94.40 16 98.00 98.00 98,00 9 10 11 97.45 6 98.75 99.40 12 98.65 5 99.25 7 99.60 8 97.80 89.55 98.0 99.0 99.0 100.90 1 2 3 4 SQURES LAID ON GROUND 99.70 99.85 98.45 CONTOURS INTERPOLATED CONTOURING 32
  • 34. 2. Indirect Method: (Square Method) The important points within the squares may be taken when required and located by measurements from the corners. The squares are plotted and the reduced levels of the corners are written on the plan. 100.4 97.60 96.05 94.40 98.00 98.00 98,00 97.45 99.25 98.65 99.60 98.75 97.80 99.40 89.55 98.0 99.0 99.0 100.90 99.70 99.85 98.45 SQUARE METHOD CONTOURING 33
  • 35. 2. Indirect Method: By Cross- Sections: This method is most suitable for the survey of long narrow strips such as a road, railway or canal etc. 70 RD 580 70.6 RD 560 70.8 70 69.1 69 69 68.8 69.1 70.8 70.2 69.1 70.4 70.5 70.8 66.3 70.6 70.8 71 RD 540 71.2 67 68 71 69 RD 520 71.6 71.2 70.6 70 71 CONTOURING 71 72.4 71.7 Fig. X-Section Method 34
  • 36. 2. Indirect Method: By Cross- Sections: This method is most suitable for the surveys of long narrow strips such as a road, railway or canal etc. Cross sections are run transverse to the centre line of the work and representative points are marked along the lines of cross-section. The cross-section lines need not necessarily be at right angles to the centre line of the work. This may be inclined at any angle to the centre line if necessary. The spacing of the cross-sections depends upon the topography of the country and the nature of the survey. CONTOURING 35
  • 37. 2. Indirect Method: By Cross- Sections: The common value is 20 to 30 m in hilly country and 100m in flat country. The levels of the points along the section lines are plotted on the plan and the contours are then interpolated as usual as shown in the fig. CONTOURING 36
  • 38. 2. Indirect Method: Fig. Stadia Wires at Diaphragm (iii) By Tacheometric method: A techeometer is a transit theodolite having a diaphragm fitted with two stadia wires, one above and other below the central wire. The horizontal distance between the instrument and staff station may be determined by multiplying the difference of the staff readings of the upper and lower stadia wires with the stadia constant of the instrument, which is usually 100.Thus the techeometer is used for both the vertical as well as horizontal measurements. CONTOURING 37
  • 39. 2. Indirect Method: iii) By Tacheometric method (contd…): This method is most suitable in hilly areas as the number of stations which can be commanded by a techeometer is far more than those by a level and thus the number of instrument settings are considerably reduced. A number of radial lines are laid out at a known angular interval and representative points are marked by pegs along these radial lines. Their elevations and distances are then calculated and plotted on the plan and the contour lines are then interpolated. CONTOURING 38
  • 40. INTERPOLATION OF CONTOURS The process of spacing the contours proportionally between the plotted ground – points is termed as interpolation of contours . This becomes necessary in the case of indirect contouring as only the spot levels are taken in this method. While interpolation of contours the ground between any two points is assumed to be uniformly sloping. CONTOURING 39
  • 41. INTERPOLATION OF CONTOURS There are three main methods of interpolation: i) By Estimation: The position of the contour points between ground - points are estimated roughly and the contours are then drawn through these points. This is a rough method and is suitable for small scale maps. ii) By arithmetical calculation: This is very tedious but accurate method and is used for small areas where accurate results are necessary. The contours are interpolated as under: CONTOURING 40
  • 42. INTERPOLATION OF CONTOURS ii) By arithmetical calculation: Suppose A and B are two points at a distance of 30 m and the reduced level of A and B are 25.45m and 27.54m respectively .Taking the contour interval as 1m, 26 and 27 m contours may be interpolated in between A and B. The difference of level between A and B is 2.09m.the difference of level between A and 26m,and A and 27m is 0.55mand 1.55 m respectively. Therefore the horizontal distance between A and 26 m contour =0.55/2.09 x 30m and Between A and 27 m contour =1.55/2.09 x 30m. These distances are then plotted to scale on the map. CONTOURING 41
  • 43. INTERPOLATION OF CONTOURS Graphical method of (iii) By Graphical method: interpolation is 65 simpler as compared B 62.5m to arithmetical 2 method and also the 60 60m results obtained are accurate. Out of several graphical 55 1 55m methods, the most common is as given A 51.5m below: 50 CONTOURING 42
  • 44. Graphical method: As shown in the fig. suppose the contour interval is 5m, then on a piece of tracing cloth, a number of parallel lines spaced at 0.5 m (usually 1/10th of the contour interval) are drawn. Every tenth line being made thick. Suppose it is required to interpolate contours between two points A and B of elevation 51.5m and 62.5m respectively. CONTOURING 43
  • 45. INTERPOLATION OF CONTOURS (iii) By Graphical method: If the bottom line represents an elevation of 50m. Then the successive thick lines will represent 55m, 60m and 65m, etc. Place the tracing cloth so that the point A is on the third line from the bottom, now move the tracing cloth until B is on the fifth line above the 60m thick line. The intersection of the thick lines 1 and 2 representing elevations of 55m and 60 m and the line AB give the position of the points on the 55m and 60m contours respectively and are pricked through on the plan with a pin. CONTOURING 44
  • 46. DRAWING THE CONTOUR LINES Contour lines are drawn as fine and smooth free hand curved lines. Sometimes they are represented by broken lines .They are inked in either in black or brown colour. A drawing pen gives a better line than a writing pen and French curves should be used as much as possible .Every fifth contour is made thicker than the rest. The elevation of contours must be written in a uniform manner, either on the higher side or in a gap left in the line .When the contour lines are very long, their elevations are written at two or three places along the contour .In the case of small scale maps, it is sufficient to figure every fifth contour. CONTOURING 45
  • 47. USES OF CONTOUR MAP (i) A contour map furnishes information regarding the features of the ground , whether it is flat, undulating or mountainous. (ii) From a contour map , sections may be easily drawn in any direction (iii) Intervisibility between two ground points plotted on map can be ascertained (iv) It enables an engineer to approximately select the most economical or suitable site for an engineering project such as a road, a railway, a canal or a pipe line etc. (v) A route of a given grade can be traced on the map. (vi) Catchment area and capacity of a reservoir may be determined from the contour map. (vii) Contour map may be used to determine the quantities of earth work. CONTOURING 45
  • 48. HOME ASSIGNMENT Q.No.1. (a) Define contour. (b) What is a contour interval and on what factors does it depend?. Q.No.2.(a) What is difference between a contour interval and horizontal …………….equivalent?. (b) Suggest contour intervals for following :(i) A hill Survey (ii) A city Survey (iii) survey of a dam site. Q.No.3. Describe with neat sketches the characteristics of contours. Q.No.4. Show contours to represent the following :(i) A Hill (ii) A Depression (iii) A valley (iv) A vertical cliff (v) A saddle. Q.No.5. Describe various methods of interpolation of contours. CONTOURING 46